EVOLUTION AND INTERACTION OF MIGRATING CROSS-ARC VOLCANISM AND BACKARC RIFTING - AN EXAMPLE FROM THE SOUTHERN HAVRE TROUGH (35-DEGREES-20'-37-DEGREES-S)

The active southern Havre Trough (35 degrees 20' - 37 degrees S) backa rc basin is interpreted to form from the evolution and interaction of migrating cross-are magmatism, and the progressive development of long itudinal rift grabens. The proposed migration of the proto-Kermadec ar e front from the Colville are at similar to 5 Ma to the active Kermade c are margin is recorded by the construction of are edifices trailed a cross the intervening, and contemporaneously rifting, backarc complex. Migration trails are identified for at least four are volcano sources . The Rumble V are migration trail is the most prominent, forming a co ntinuous, high-standing, magmatic are ridge. These postulated are trai ls segment the backarc region and initially limit rift development to the intervening proto-are crust blocks. Early rifting between the are volcanoes forms fully developed rift grabens which, with progressive b asin widening, propagate longitudinally across the trails of migrating constructional are magmatism. A model of the balance between rates of constructional are magma production M(R) and destructive back-are (ri fting) extension V-E is proposed. When V-E is high, M(R) is insufficie nt to keep abreast of destructional rifting, resulting in small, isola ted are massifs quickly dismembered by rifling. Conversely, when V-E i s low, M(R) is sufficiently greater than rifting to produce a continuo us, high-standing cross-are ridge which segments longitudinal rift dev elopment. Migrating are magmatism will be best observed, and preserved , in rifting backarc basins when M(R) is > 600 km(3) m.y.(-1) 100 km(- 1) of plate boundary and V-E is < 25 mm a(-1).